Miranda IM is a light-weight, open-source instant messenger that can connect to tons of IM networks, including ICQ, AIM, MSN, Jabber and Yahoo. In addition to IM networks, Miranda also supports IRC.

Software supports all the basic features available on any IM nowadays, but its true beauty is in its range of plugins -- there are over 120 open-source plugins available for Miranda that extend its capabilities when necessary. Having said that, most users never need those plugins -- just like they don't need most of the features bundled with proprietary IM tools, such as MSN or AIM.